@Flohack Well, I'm not worried by Qt itself: it's generally well-documented (property and method deprecations too), so anything can easily (in general) be set up to prevent issues.
The problem comes when we take in account UT-specific code, which makes usage of Qt private imports. I see, running 'grep' on my local folder where I keep all the UT code, that more than 10 components depend on them - including Unity8, the web browser, the Ubuntu UI Toolkit, the Ubuntu keyboard, and some Mir helper.
It means this software should be heavily tested in order to check if there's any regression. This is what Canonical used to do anytime they were moving to a newer version - IIRC they had to fix a few things when they moved to Xenial which uses Qt5.6.
A (partially) good news is that everything should have been already tested on Qt 5.6 and it should just work "out-of-the-box". Anyway I don't think there's any value in moving from the current patched version of Qt 5.4.x to another - as long as we stay on vivid, Qt 5.4 is not the problem.
I think this should be one of the main discussions after we've started to work on Xenial. Qt version, UITK, Unity8, Mir, and the package format are things extremely related each other. The only thing that I can say for sure is that it wouldn't be a problem if we were on Snaps, Flatpaks or Appimages, since they bundle a specific version of the required libraries... 😕
